The default format for new mails are now html. There is no way to switch back to plain text as default.
kmail2 5.5.3 in Ubuntu 17.10 and kmail2 5.7.40 alpha in neon devedition unstable both have the option "Prefer HTML to plain text" on the security page of the settings dialog and the menu item Folder->Message Default Format with the options Prefer HTML to Plain Text, Prefer Plain Text to HTML and Use Global Setting. What is your version of kmail/distribution? Do you have these settings?
Version is kmail 5.6.3 , neon user edition on ubuntu 16.04. I checked all these setting without success. I also tried to deactivate the html-toolbox with the toolbox settings. But the rich-text button is allways active when creating a new mail. Replies to plain text messages works as expected.
fixed in 5.7.0